Paul wants his readers to feel the weight of an argument they can finish themselves. God has already given the greatest gift He could give, His own Son. Barnes calls it "the highest proof of love" and reasons that "He that has given the greater gift will not withhold the less." If God did not hold back His Son, He will not hold back anything His people truly need. The question is built so that the only possible answer is: He will.

The question answers itself: having given His Son, God will surely give the rest. Paul then moves straight into more questions that build on this one: "Who shall lay any thing to the charge of God's elect?" (Romans 8:33) and "Who is he that condemneth?" (Romans 8:34). He ends with the settled conviction that nothing "shall be able to separate us from the love of God, which is in Christ Jesus our Lord" (Romans 8:39).

"Spared not" means God did not hold His Son back from suffering and death. "Delivered him up" is the same idea Paul used earlier: Jesus "was delivered for our offences, and was raised again for our justification" (Romans 4:25). Gill explains that the Son was given up "as their surety and substitute, in their room and stead," meaning He took the place of His people. The word "freely" stresses that the "all things" come as a gift of grace, not as wages earned.

This question shows what God is like by pointing to the cross. Matthew Henry writes that "All things whatever, in heaven and earth, are not so great a display of God's free love, as the gift of his coequal Son to be the atonement on the cross for the sin of man." Love that has already given the most will not refuse the less.

Notice the words "with him." The "all things" are not given apart from Christ but along with Him. God does not hand out blessings as a substitute for His Son; He gives His Son, and everything His people need comes with Him.